I am having trouble creating a calculated field to filter some data in a project I am working on.

FYI i cannot share the workbook since it has Private Info

So, here's the issue.... I have multiple loan numbers (data example below), I need to filter any repeating loan numbers to take the least severe rating, the ratings have no number scale to identify them and when I try to make a scale it doesn't work because "Good" ratings are automatic so Tableau treats them as "Null" in calculations even though they say good if I look at the underlying data..... so in the example below for the loan # 444 I would want to filter so only the Good rating is shown in my graph, thanks for any help I receive in advance!